This invention relates to a plane antenna utilizing microstrip lines and, especially, to a crank-shaped microstrip line antenna having the number of elements reduced for obtaining directivity in a slanting direction and reducing its size.
As shown in FIGS. 9 and 10 of the Japanese patent opening gazette No. 57-99803 or FIGS. 10 and 11 of the corresponding U.S. Pat. No. 4,475,107, a typical example of the crank-shaped microstrip line antenna is composed of a pair of conductors each having relatively long hill portions and relatively short valley portions which are connected alternately. The two conductor lines which form the pair are arranged in parallel in such a relationship in that each valley portion of one conductor line faces to the middle of each hill portion of the other. Each part of the pair of conductor lines having a length corresponding to the sum of one hill portion and one valley portion constitutes an antenna element for circularly or linearly polarized radiation of electromagnetic wave corresponding to twice the wavelength thereof. Accordingly, the antenna as shown in the abovecited drawings consists of three elements.
As the conductor lines are formed on a dielectric substrate, the wavelength of the electromagnetic wave on the conductor lines differs from the wavelength in space in correspondence with the dielectric constant .epsilon. of the substrate even at the same frequency. For example, the wavelength of an electromagnetic wave on a conductor line formed on a polyethylene substrate (.epsilon.=2.5) is reduced to about 63% of the wavelength in space and the wavelength of the electromagnetic wave on a conductor line formed on a foamed polyethylene substrate (.epsilon.=1.7) is reduced to about 80% of the wavelength in space.
In the above-mentioned crank-shaped microstrip line antenna, the main beam of radiation is directed normally to the antenna plane when the length of each conductor line in each antenna element corresponds to twice the wavelenqth of the electromaqnetic wave. Such directivity is referred to as "broad side type". However, the main beam of radiation is directed to a slanting direction when the length of each portion of the crank-shaped conductor is expanded in the longitudinal direction of the microstrip line. Such directivity is referred to as "side looking type".
When one intends to receive an electric wave from a stationary artificial satellite in a region of middle or high latitude with a parabolic antenna or a plane of broad side type, it is necessary to raise the antenna very much from the horizontal plane so as to put the antenna aperture plane or the antenna plane normally to the incoming direction of the electric wave. This results in the antenna being subjected to an increased wind pressure when the antenna is disposed on the roof of a running vehicle. However, this wind pressure should be reduced with a plane antenna of side looking type having suitably slanted directivity, since it can effect reception where in a nearly horizontal attitude.
A conventional microstrip line antenna includes about ten crank-shaped antenna elements connected in series. Although the gain of the antenna rises with increase of the number of these elements, the frequency bandwidth becomes narrow. On the contrary, the frequency bandwidth increases and the gain decreases with reduction of the number of serial crank-shaped antenna elements. Accordingly, it has been a general practice that a patch antenna element is added to the end of each line of elements for improving the gain in case of the antenna of broad side type having fewer crank-shaped antenna elements.
If one intends to provide the prior art crank shaped antenna as shown in the above mentioned patent with a side looking property, it will be necessary to largely increase the length of each antenna element. For example, when the direction of radiation of the main beam is slanted by 28 degrees, the length of each antenna element viewed from this direction is reduced only by a factor of 0.88 or cos 28.degree.. In practice, however, the direction of radiation can not be slanted by 28 degrees unless the length of each antenna element is increased by a factor of 1.5. This results in significant reduction in the number of antenna elements which can be arranged in series and consequent reduction in the antenna gain.
Although the main beam of radiation of the electric wave to be used is directed to a direction slanted by 28 degrees when the length of each antenna element is expanded by a factor of 1.5, for example, in order to obtain the side looking property, it is necessary to pay attention to the fact that the main beam of the electric wave having a wavelength increased by a factor of about 1.5 is radiated to a direction nearly vertical to the antenna plane. For the same reason, electric waves having wavelengths which are 1.0 to 1.5 times the wavelength of the electric wave to be used are radiated to respective directions between zero and 28 degrees. It is also understood that an electric wave having a wavelength shorter than that of the electric wave to be used is radiated to a direction slanted by much more than 28 degrees. It will be understood also in case of the crank-shaped microstrip line antenna of broad side type that undesirable electric waves having shorter wavelengths than the electric wave to be used and radiated normally to the antenna plane are radiated in slanting directions.
Accordingly, a first object of this invention is to suppress the electric wave radiation of undesirable wavelength directed to undesirable direction to improve the signal-to-noise ratio of the antenna.
As described above, in the microstrip line antenna of broad side type, the gain reduction due to the reduced number of serial antenna elements can be compensated by the addition of patch antenna element to the end of each line. However, it is difficult for the patch antenna element to reduce the energy radiated to the front direction by the phase difference to obtain the side looking property since it has a large gain only in the front direction. Therefore, it is ineffective as a countermeasure to the reduction of antenna elements effected for providing the crank-shaped antenna with the side looking property.
Accordingly, a second object of this invention is to provide a crank-shaped microstrip line antenna having relatively few elements, especially, a crank-shaped antenna having improved antenna gain and aperture efficiency and consequent inprovement in radiation efficiency of each antenna element and in directivity gain regardless of the number of antenna elements reduced for obtaining the side looking property.